1. Field of the Invention
The present invention relates to an apparatus for filtering liquids, such as fibre pulp suspensions in the paper making industry.
2. Description of the Prior Art
A common type of filtering apparatuses for filtering liquids comprises at least one rotatable annular hollow filter disc with two opposed side walls of filter material adapted to be at least partly immersed in a body of the liquid to be filtered. Within the hollow filter disc, there is a plurality of circumferentially spaced support arms, for supporting the side walls of filter material.
This type of filtering apparatus is relatively expensive, and, therefore, in practice, it has been used to a limited extent in the paper making industry for the purpose of concentrating fibre suspensions up to about 3% fibre content. For that purpose, it has been common practice to use a rotary filtering apparatus of the so-called drum type, which has a horizontal cylindrical drum, the circumferential wall of which is covered with filter material. Such a drum filtering apparatus is simple and inexpensive. However, a drum filtering apparatus is much more space demanding in installation, as compared with a disc filtering apparatus of a corresponding capacity.
Conventionally, a filter disc is an assemblage of a great number of separate complicated constructed sector elements. Each sector element includes two opposed perforated plate walls for supporting the side walls of filter material, such as cloth. These perforated plates substantially decreases the operative filtering area of the filter material, since there must be sufficient broad wall portions between the holes in the plates, in order to make the plates strong. Such a decreased operative filtering area is a serious deficiency in connection with filtration of the kind of fibre suspensions which suffers from poor dewatering properties (Low-freeness suspensions).
Attempts to replace such perforated plates by prefabricated steel net, in order to increase the operative area of the filter material, have proved unsuitable for reasons of quality and costs. A drawback to the use of steel nets is that each end of the wires of such a net has to be welded to the filter disc. Also, to cover the entire side of a filter disc with steel net, several pieces of steel net must be jointed to each other by welding abutting ends of wires onto reinforcing steel strips located at the inner side of the steel net. In addition, each welded end of wire or joints of wires must be covered with some kind of protective means, to ensure that the filter material will not be damaged by the welded ends of wires. Application of reinforcing steel strips and protective means on the filter disc would mean, that the operative area of the filter material would be decreased. The resulted great number of welded wire ends would increase the risk of arising defective welded ends of wire, which in turn would result in stoppage. Besides, by experience it is known that the use of net structures for supporting a filter cloth results in an increased wear and tear of the filter cloth.